This invention relates generally to purification of a gas stream and more particularly to the purification of a mixed gas stream of hydrogen and hydrogen selenide.
As is known in the art, hydrogen selenide is an important intermediate component in the production of zinc selenide which is used as an optical material at infrared wavelengths.
It is also well known in the art that hydrogen selenide is an extremely toxic gas. Current techniques for using hydrogen selenide in the production of zinc selenide optical material involve directing gas vapors from a bulk source of H.sub.2 Se into a chemical vapor deposition reactor to effect a chemical reaction to form zinc selenide. It would be desirable to reduce exposure to such a toxic substance by manufacturing hydrogen selenide in a continuous process and on an "as needed" basis, so that in the event of an accident, the system can be immediately shutdown in a controlled manner permitting little or no hydrogen selenide gas to escape into an external environment. This scenario would be contrasted to an industrial accident where, for example, a valve was sheared off of a tank storing H.sub.2 Se allowing for an uncontrolled escape of the gas.
Several processes for the production of hydrogen selenide are known in the art. Commercially, H.sub.2 Se is typically formed by the reaction of an acid and metal selenide, for example, HCl+FeSe. This gas generation process produces impurities such as H.sub.2 O and HCl in the desired gas product stream which must be removed to produce a suitably pure gas source for chemical vapor deposition of ZnSe. Moreover, this process is also a batch-type process.
Another such technique is described in Great Britain Patent Application No. 1508749 and includes the step of bubbling hydrogen gas through a molten source of selenium to produce hydrogen selenide. This reaction is a low yield reaction. After the hydrogen selenide is formed and leaves the reactor vessel, the vapor stream includes a substantial amount of unreacted hydrogen. However, a gas vapor stream, typically of at least 90% pure hydrogen selenide is required to provide a suitable reactant during chemical vapor deposition of zinc selenide.
To increase the purity of H.sub.2 Se, generally the hydrogen selenide+hydrogen vapor stream is passed through a batch-type cryogenic process to separate the hydrogen selenide from the hydrogen. The process includes introducing the mixed vapor stream from the reactor vessel into a first vessel cooled to a temperature at which the hydrogen selenide freezes out of the mixture, while allowing the gaseous hydrogen to escape from the first vessel until a quantity of the solid hydride is accumulated in the first vessel. The first vessel is then disconnected from the reactor vessel and connected to a second vessel also cooled to a temperature at which the hydrate freezes. The first vessel is allowed to rise to a temperature at which the hydride therein develops a gaseous phase so that it is transferred to the second vessel. The hydrate is then re-solidified in the second vessel as hydrogen is again allowed to escape from the second vessel.
There are also several problems with this latter approach. A first problem is that the process is non-continuous (i.e. a batch-type process). Inherent in this process is a requirement for storage for long periods of time of a large quantity of hydrogen selenide in either a gaseous, liquid, or solid form. Since hydrogen selenide has an extremely high vapor pressure, the presence of stored hydrogen selenide during this cryogenic separation process presents potential safety problems in the event of an accident. A second problem with such process is the complexity and expense involved with the technique. These factors limit the availability of hydrogen selenide production to only gas suppliers or large users of the product. Small end user such as ZnSe optical material manufacturers must depend upon tanks of such gases from suppliers. This, however, increases the possibility of an industrial accident.